Enterococcus faecium, a lactic acid bacterium (LAB), was evaluated for its ability to remove copper(II) ions from water. The effects of the pH, contact time, initial concentration of copper(II) ions, and temperature on the biosorption rate and capacity were studied. The initial concentrations of copper(II) ions used to determine the maximum amount of biosorbed copper(II) ions onto lyophilised lactic acid bacterium varied from 25 mg L-1 to 500 mg L-1. Maximum biosorption capacities were attained at pH 5.0 and 6.0. Temperature variation between 20°C and 40°C did not affect the biosorption capacity of the bacterial biomass. The highest copper(II) ion removal capacity was 106.4 mg per g dry biomass. The correlation regression coefficients show that the biosorption process can be well defined by the Freundlich equation. The change in biosorption capacity with time was found to fit a pseudo-second-order equation.

Copper has been one of the most widely used metals for centuries and is mainly employed in electrical and electroplating industries. Because of its toxicity to living organisms, its presence in the environment causes serious toxicological concerns. Copper is known to deposit usually in the brain, skin, liver, pancreas and myocardium (Davis et al., 2000). Wastewaters of especially the electrical and electroplating industries contain high levels of Cu2+ ions and treatment of such waters to remove Cu2+ ions is needed before disposal.

Because of their nature and membrane compositions, bacterial biomasses are natural adsorbents for metals. The heavy metal uptakes involve the passage of the metal species into the cell across the cell membrane during the cell metabolic cycle. This mode of metal capture is referred to as active uptake (Kapoor et al., 1999). If one considers that nonviable biomass is not biologically active, its metal uptake can be regarded as a passive adsorption process and, thus, be correlated with mathematical sorption models as the Langmuir and Freundlich equations (Cruz et al., 2004). Furthermore the bacterial surfaces contain polarizable groups (sites) such as phosphate, carboxyl, hydroxyl and amino groups, which are capable of interacting with cations. These sites may also contribute to the reversible metal binding capacity of the biomass (Liu et al., 2003). Therefore, biosorption is a process that uses any biomass to sorb ions from aqueous solutions.

In the present work, the biosorption of copper(II) ions onto a lactic acid bacterium, namely Enterococcus faecium, was studied by investigating the influences of different experimental parameters such as pH, contact time, initial concentration of copper(II) ions, and temperature on the rate and capacity of the biosorption.

MATERIALS AND METHODS

Growth Media and Culture Conditions

The strain used in this study was Enterococcus faecium, a lactic acid bacterium, isolated from meat and identified using the RiboPrinter® Microbial Characterization System (DuPont). Bacterial isolate was inoculated into MRS-broth for 48 h at 37°C. Then the biomass was centrifuged (8000xg, 15 min), washed twice with ultra-pure water (Milli-Q), lyophilized and stored at 20°C.

Copper(II) Ion Solutions

A stock solution of Cu2+ ions used in this study was prepared by dissolving an accurate quantity of copper(II) nitrate in deionized water. Other concentrations prepared from this stock solution by dilution varied from 25 mg L-1 and 500 mg L-1 and the pH of the working solutions was adjusted to the desired values with 0.1 M HCl or 0.1 M NaOH. Fresh dilutions were used for each experiment. All the chemicals used were of analytical grade.

Biosorption Experiments

The biosorption of Cu2+ ions onto the lyophilized biomass from aqueous solutions was investigated in batch experiments. The experiments were carried out with 50 mg of the biosorbent in 100 mL of a Cu2+ ion solution in 250 mL bottles at 30°C on an orbital shaker operating at 150 rpm. The effect of pH on the biosorption rate was investigated in the pH range of 1.0-6.0. The pH values of the medium were adjusted to the desired value by adding HCl or NaOH at the beginning of the experiments and were not controlled further. The initial concentrations of Cu2+ ion solutions varied from 25 mg L-1 to 500 mg L-1. The effect of temperature (20-40°C) on the biosorption capacities of the biosorbent was determined at pH 5. After the contact time, the solutions were centrifuged at 5000 rpm for 5 min and the supernatants were used to determine the concentration of nonadsorbed Cu2+ ions.

The concentrations of the nonadsorbed Cu2+ ions in the supernatants were determined by using a flame atomic absorption spectrophotometer (Perkin Elmer A. Analyst 800 Model) with an air-acetylene flame. The instrument response was periodically checked using a standard Cu2+ ion solution.

The equilibrium sorption capacity of the E. faecium biomass at the corresponding equilibrium conditions was determined using the following mass balance equation:

where qe is the amount of Cu2+ ions adsorbed onto the biomass (mg g-1), Co is the initial Cu2+ ion concentration in solution (mg L-1), Ce is the final metal ion concentration in solution (mg L-1), V is the volume of the medium (L) and m is the amount of the biomass used in the adsorption process (g).

RESULTS AND DISCUSSION

Figure 1 shows the effects of contact time for the biosorption of Cu2+ ions onto the lyophilized biomass of E. faecium. The experimental conditions are indicated in the figure legend. A rapid sorption rate was observed within the first 30 min of the biosorption process and then a considerable increase in the uptake capacity of the biosorbent for Cu2+ ions continued up to 600 min. Thereafter, it remained nearly constant. This rapid initial uptake is similar to previous reports on the biosorption of heavy metals by different biosorbents (Puranik and Paknikar, 1997; Yin et al., 1999). The subsequent biosorption experiments using the lyophilized biomass of E. faecium were performed with a contact time of 24 h.

Earlier studies on heavy metal biosorptions have shown that pH was the single most important parameter affecting the biosorption processes because pH influences both cell surfaces (metal binding sites) and metal chemistry in the processes. Several researches have investigated the effect of pH on biosorption of heavy metals using different kinds of microbial biomass and found that maximum biosorption occurred around neutral pH (Feng and Aldrich, 2004; Xue et al., 1988).

To find the optimal pH for the effective biosorption of Cu2+ ions onto E. faecium, we performed experiments at different initial pH values varying from 1.0 to 6.0 (Figure 2). As expected, the Cu2+ ion removal process was a strongly pH-dependent process and the two highest uptake values were obtained at pH 5 and 6. These findings are similar to those for various biosorption processes reported in the literature. The Cu2+ ion removal efficiency of E. faecium dropped sharply at pH≤4. To avoid precipitation of Cu2+ ion as its hydroxide, pH values higher than 6 were not tested for the biosorption. Based on these results, a pH value of 5.0 was used in further experiments.

The temperature of the adsorption medium could be important parameter for energy-dependent mechanisms of metal adsorption by microbial cells. Equilibrium uptakes of Cu2+ ions onto the E. faecium biomass at different temperatures (20-40°C) are shown in Figure 3. The results showed that the temperature did not alter significantly the Cu2+ ion uptake capacity of the biosorbent. Nearly constant values of qe were obtained at the temperatures applied. Thus, 30°C was chosen as the temperature for further experiments.

In Figure 4, the Cu2+ ion adsorption capacities of the lyophilized biomass of E. faecium were presented as a function of the initial concentration of metal ions within the adsorption medium. Up to 250 mg L-1 initial concentration, the amount of adsorbed Cu2+ ions per unit mass of the biomass increased. This is a reasonable observation because there are many binding sites available on the biosorbent for Cu2+ ions at low concentration. Beyond the concentration of 250 mg L-1, the adsorption capacity of the biosorbent remained relatively constant. This was possibly due to saturation of metal binding sites on the biosorbent at concentrations higher than 250 mg L-1. Further experiments were carried out at 250 mg L-1 initial concentration of copper ions.

Langmuir and Freundlich isotherms were used to analyze the biosorption data (Langmuir, 1918; Freundlich, 1906). The Langmuir isotherm model is valid for monolayer adsorption on to surface containing a given number of identical sorption sites. The Langmuir equation is commonly expressed as:

where qe and qmax are the amount of metal ion removed and maximum uptake capacity (mg g-1), respectively, Ce is the equilibrium concentration (mg L-1), and KL is the Langmuir constant.

The Freundlich equation assumes the sorption on a heterogeneous surface and is described by the following equation:

where KF and n are Freundlich constants.

The Langmuir and Freundlich isotherm parameters calculated from the corresponding linear plots are given in Table 1. It appears that the Freundlich isotherm gives a better fit than the Langmuir isotherm. The regression coefficient value of the Freundlich equation (0.985) is better than that of the Langmuir equation (0.966). The plot of the observed equilibrium adsorption and the Langmuir and Freundlich isotherms is given in Figure 5.

Having carried out the studies to determine the efficiency of Cu2+ biosorption on E. faecium, the pseudo-first-order and pseudo-second-order kinetic models were applied to the experimental data to elucidate the biosorption mechanism.

where qe and qt are the amounts of the Cu2+ ions absorbed at equilibrium and at time t (mg g-1) respectively, and k1 is the pseudo first-order rate constant (min-1). Values of k1 and q1 were calculated from the plot of log (qe-qt) against t.

where q2 is the maximum adsorption capacity of the biomass (mg g-1) for pseudo-second-order adsorption, k2 is the rate constant for the pseudo-second-order adsorption (g mg-1 min-1). Values of k2 and q2 were calculated from the plot of t/qt against t (Figure 6). It was found that the pseudo-second-order kinetic model fit better to the experimentally obtained data then the pseudo-first-order kinetic model. The pseudo-second-order kinetic model parameters for the adsorption of Cu2+ ions onto the biosorbent are given in Table 2.

CONCLUSION

In this study, the biosorption of Cu2+ ions from aqueous solutions onto a lyophilized lactic acid bacterium, Enterococcus faecium, was investigated. Experiments were performed by varying the pH, initial metal ion concentration, contact time, and temperature. The maximum adsorption capacity of biosorbent was found to be 106.38 mg g-1 under conditions of pH 5.0, contact time of 24 h, biosorbent mass of 50 mg and initial metal ion concentration of 250 mg L-1. For the biosorption process, the results indicated that the adsorption equilibrium data fitted with the Freundlich isotherm model. The biosorption process was found to conform to a pseudo-second-order kinetic equation. It can be concluded that the lyophilized lactic acid bacterium, Enterococcus faecium, has a potential to be used as an alternative biosorbent material for the removal of Cu2+ ions from aqueous solution.